1. Technical Field
The present invention relates to a high precision optical scanner which can be produced at relatively low cost for use in a wide variety of both military and civilian applications.
2. Discussion of Related Art
Optical scanner systems generally comprise a source of electromagnetic radiation, a reflective member and a detector. The source, such as a laser modulator, generates a beam of electromagnetic radiation which impinges upon the reflective member. The reflective member deflects the beam, typically toward some target object. The detector receives the deflected beam after it has made contact with the target object or objects. Movement of the reflective member, such as a mirror, defines a scan pattern of the optical scanning system.
There are many different techniques for implementing a desired scan pattern, although these techniques can be grouped into a few broad categories. A first well-known group of scanners are designed to operate in a resonant mode. These scanners are supported by a resilient member, such as a flexure or a spring. The resiliency of the spring and the shape and mass of the mirror define a natural frequency. If driven at this natural frequency, this type of scanner requires very little power.
In resonant scanning systems, the mirror typically vibrates back and forth about a neutral position and the angular displacement of the mirror follows a non-linear sinusoidal pattern. Hence, these scanners are typically used when bi-directional scanning is required. The non-linearity in the scan pattern and other limitations generally render this type of scanner ill-suited for applications where highly precise scanning is required.
A second variety of scanners do not necessarily rely on a natural resonant frequency of the mirror and spring structure. For instance, such a scanner may comprise a mirror which scans in a single direction, making complete rotations about a central axis of rotation. The mirror can have a single reflective face or plural reflective faces. Other scanners rotate about a central rotation axis, but only through a limited arc. This type of scanner can scan in two directions or a single direction. In the later category, the scanner may rotate the mirror at a prescribed constant velocity over an angular range of positions, and then the scanner quickly returns the mirror to its initial starting position to start another scan. This type of scanner is typically referred to as a scan-flyback or scan-retrace scanner.
Scan-flyback scanners are preferred in many military applications where a high degree of precision is required. Single-direction scanners are superior to bi-directional scanners because it is difficult to ensure constant revisit times throughout the image with bi-directional scanners. With bi-directional scanners, the end portions of the image are formed by readings that are collected close together in succession, but there is a relatively long time between consecutive pairs of such close readings, whereas the central part of the image is visited at a relatively constant rate. This variance in the revisit times at different positions along the range of the scan is undesirable from an image processing standpoint.
Both of the above-mentioned types of scanners typically employ a control mechanism which uses some type of feedback. For instance, the scanners may use a sensor which detects the location of the mirror, or some other position-related parameter. Signals from the sensor are input to the motor's controller as feedback, and corrective action is performed on the basis of these signals. The complexity of this feedback mechanism varies with the type of scanner and the scanner application (e.g., scanners which require highly precise scan profiles may require complex feedback mechanisms).
The design of an effective scanner presents a number of challenges. High precision in scanning is always desirable, and may be a mandatory design criterion in some applications. Generally, distortion produced in a scanning operation may be attributed to two basic sources: (a) the structural components of the scanner itself (e.g., the mirror, resilient components, mounting structure, etc.); and (b) the motor and motor control mechanism. For instance, with respect to the first source of errors, movement of the mirror may establish inertial forces which may cause aberrations in a desired scan profile. Also, the manner of applying force to the mirror may create forces within the mirror itself which cause distortions in the scan profile. For instance, some scanners apply force to the mirror at a first position on the mirror and provide a counteracting resilient force to the mirror at a spatially separate location on the mirror. This may be implemented by some type of piston or cam which contacts the mirror at the first location on the mirror, and a spring which contacts the mirror at the second location on the mirror. These opposing forces create bending within the mirror which distorts the desired scan pattern. These bending effects may be very small, but when high precision is required, these bending forces are enough to render the scanner ineffective.
With respect to the second source of errors, it is difficult to ensure a constant velocity in scanning with many conventional types of motors and motor controllers. As noted above, feedback mechanisms are prevalent in scanners, but feedback mechanisms are, by definition, reactive, and do not generally ensure error-free scanning. Also, the additional complexity of the feedback mechanism increases the potential of scanner failure.
The design of an effective scanner must also meet various non-technical criteria. Notably, it is desirable to produce the scanner at relatively low cost, especially in those scanners that are intended to be replaceable, or are intended to be used in low-price commercial merchandise, such as printers, display devices, and document scanners.